First, get a road map for the area in which you'll be traveling. Trace out your route. Then using the map, figure how many miles to your destination. Calculate the amount of time it will take to get there by driving the speed limits. Figure how much gasoline it will take and how much that gasoline will cost.
As you're actually taking your trip, keep track of your mileage, time, gallons of gasoline, and costs. Compare them to your estimates. How did you do? If your calculations were correct, the actual amounts should be very close to your estimates.
Do this several times over the course of the summer. You should get better at it, while improving your map and math skills. And remember to enjoy yourself on your trip.